New Bike! Looking for some tuning advice...
- Thread starter jdschwass
- Start date
So good news and bad news... good news is that my jets arrived! Bad news is they ain't the right jets. I ordered both Mikuni and Keihin so I'd have a good stock, and the pilots are all too big. They are the same, just bigger. It said they fit the VM, TM, and TMX carbs, and mine is supposedly a clone of the VM22...
Thankfully I have a new carb arriving tomorrow, so I can swap the whole thing over. But I would still like to get the old carb fixed up so I can use it down the road.
I found the attached pic online, and comparing my jet, it does not have that flared seat at the base of the jet, and also has the extended brass tube on the top. Visually, I'd say mine is a CR/YZ style. The VM28 is also close, but it's missing the chamfer cut on the top of the threads, and the narrow section looks slightly smaller.
Unfortunately I buggered up the jet pretty good, so I can't take any accurate measurements.
I'd love to get your $0.02 before I keep shopping!
Many thanks.
EDIT: Looking at another pic, the jet for the VM28 looks to be pretty close.
Thankfully I have a new carb arriving tomorrow, so I can swap the whole thing over. But I would still like to get the old carb fixed up so I can use it down the road.
I found the attached pic online, and comparing my jet, it does not have that flared seat at the base of the jet, and also has the extended brass tube on the top. Visually, I'd say mine is a CR/YZ style. The VM28 is also close, but it's missing the chamfer cut on the top of the threads, and the narrow section looks slightly smaller.
Unfortunately I buggered up the jet pretty good, so I can't take any accurate measurements.
I'd love to get your $0.02 before I keep shopping!
Many thanks.
EDIT: Looking at another pic, the jet for the VM28 looks to be pretty close.
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There's a site called Jets r Us and their jets pages for genuine Mikuni and Keihin jets and carb type can help you with length and width dimensions as well as thread pitch. Very helpful for figuring out which one you're holding and which one to order. They may offer info that distinguishes genuine from clone products, but I'm not sure.
Heyall, I got my new carb installed. Still waiting on the correct jets for the old one. It seems to be running pretty awesome with the stock jets. It came with a 15 pilot and 95 main. The main circuit feels great, though I'm not certain how the top end should feel. With the CVT, I can't get it over about 4300 rpm on the open road. I guess this is safe for a stock engine, but it does rip. I'm very pleased.
On the pilot circuit however, I think I am fighting a rich condition. The engine continues to run with the mixture screw all the way in, but it seems to run best when it's out around 2 turns. It doesn't seem to have a lot of impact though. The problem I'm having is that the idle speed is all over the map. I get it dialed in, go for a ride, and then it's idling over 2000 rpm. I bring it down to 1700, go for a ride, and then it's at 1300 rpm. I'm thinking the plug is getting fouled up at idle, but not certain. It idles great when cold, and then gets worse as it warms up I think. When I crack the throttle there is a brief puff of black and then it's fine.
I was thinking i'd drop a jet size whenever they arrive, but I was wondering if there is anything else I should be looking at, or if I am misreading the condition. It's so close!
Thanks for all the support on this. Much appreciated.
On the pilot circuit however, I think I am fighting a rich condition. The engine continues to run with the mixture screw all the way in, but it seems to run best when it's out around 2 turns. It doesn't seem to have a lot of impact though. The problem I'm having is that the idle speed is all over the map. I get it dialed in, go for a ride, and then it's idling over 2000 rpm. I bring it down to 1700, go for a ride, and then it's at 1300 rpm. I'm thinking the plug is getting fouled up at idle, but not certain. It idles great when cold, and then gets worse as it warms up I think. When I crack the throttle there is a brief puff of black and then it's fine.
I was thinking i'd drop a jet size whenever they arrive, but I was wondering if there is anything else I should be looking at, or if I am misreading the condition. It's so close!
Thanks for all the support on this. Much appreciated.
do you have free play in the throttle cable when the slide is down? Otherwise when you move the handlebars left to right it will be lifting the slide a bit causing idle to change. It does not sound normal for you to be able to close air/fuel adjuster and engine keeps running. Did you raise the idle slide adjuster screw by chance?
The throttle cable is perfect, just enough slack not to keep pressure on the slide. To me, I'd say having the fuel screw all the way in means that too much fuel is still passing through the pilot. Plug gets fouled up with carbon and starts to miss, hence throwing the idle for a loop.
It runs better cold, which would prefer a rich condition, once it heats up, then it's too rich.
That's my thinking anyways. I won't have the jets until Friday, so hoping to get it swapped by the weekend. Other than the idle, it's just awesome.
It runs better cold, which would prefer a rich condition, once it heats up, then it's too rich.
That's my thinking anyways. I won't have the jets until Friday, so hoping to get it swapped by the weekend. Other than the idle, it's just awesome.
That would be true for a regular Mikuni, but a chikuni the air fuel screw just meters air from the air jet and the fuel from the pilot both mixed so all that bottom screw does it meter the amount of premixed fuel and air. It does not change the ratio, only changing pilot jets does that. If you screw the needle all the way in it blocks the hole completely so in your case the fuel can only come from the hole just before the back of the slide which is why I asked if you moved the slide height adjustment from the way it came. Pics below show the air fuel path for the pilot circuit.
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Here is an older thread with the various jets sizes. Popeye had a carb with the different pilots mine are original mikuni's also the number on the flange seems to tell if they are the 15 size pilots or 30 sized . here is a snippet from that thread "
These carbs look the same and the Mikuni jets fit. There even were Mikuni marked jets installed #95 and #15 pilot. On the intake flange is also a stamp saying: 22 3847. The other carb has 5VLB 32 marked on there."
Case closed, thanks for any help. " link to carb thread (1) Mikuni VM-22 main jet thread size. | OldMiniBikes.com
These carbs look the same and the Mikuni jets fit. There even were Mikuni marked jets installed #95 and #15 pilot. On the intake flange is also a stamp saying: 22 3847. The other carb has 5VLB 32 marked on there."
Case closed, thanks for any help. " link to carb thread (1) Mikuni VM-22 main jet thread size. | OldMiniBikes.com
Sorry about that, I got a bit excited with all those pictures. I have adjusted the slide height screw to achieve the desired idle speed once I found the highest RPM while tuning the mixture. I understand the slide position impacts the the idle circuit mixture, so it has been a lot of back and forth tuning trying to get it balanced. Are you suggesting that the slide could be too high, therefore limiting the impact of the pilot circuit? I don't think this is the case, but I can certainly confirm that. Thanks, and sorry for not answering the first two times.
Yes exactly! So with the idle mixture screw all the way in, there are only 2 ways that the engine could be getting fuel to run. One is if it's pulling fuel from the hole just under the back of the slide. The slide would have to be high enough for the air velocity to be high enough to pull fuel. The other possibility is the air enricher plunger is not all the down letting a rich mixture of fuel and air in. Otherwise I'm stumped how it can run.
Just thought of another possibility, when I was taking pics I wanted to verify that with the mixture screw was all the way in it was blocking the hole and used a paper clip to feel the needle point was right at the top. At that point nothing could get thru the hole, however I did not have the spring on the needle when doing that test. It could be when spring is there it coil binds preventing the needle going all the way closed. I will check that later.
Spent some time tinkering last night. Started the bike cold at 30 degrees, checked slide and mixture and everything was where it ran the best, and idle speed set to 1800rpm. Took it for a drive, and when it got hot, it still won't idle properly. I let it continue to idle, and watched it drop down as low as 1200 before it died. Immediately removed the plug and found it to be pretty sooty. Thankfully my jets came in last night, so assuming I got the right ones this time, I plan to drop a jet size later today. Hopefully that does it.
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I don't see any soot except for the outer shell which in not a indicator of lean or rich, plug looks pretty good to me. You should be looking at the center insulator and ground strap. If the plug ground strap turns colors like blue it is running too hot. If the insulator is tan its OK, yours is mostly tan certainly not fouled which would cause erratic idling. Anyway, let us know how the changing of the pilots affects the idle.
I got out this afternoon and swapped the pilot jet to the next size smaller, going from a 15 to a 12.5 I did a bit of a drive to get it hot, and it's not better. Possibly worse? The idle mixture screw has little to no effect until it starts getting really far open, then revvs drop off. Still runs the same with the screw closed.
When driving, all is fine until I stop, then the revvs stay at a high idle for a few moments, and then start to drop.
I tried my best to capture a video of it once it was warmed up. This is with the mixture set to 1.5 turns open, and I only touched the throttle to keep it from stalling a couple of times. You can clearly see the idle won't settle down. It's got me wondering it it's a lean condition now, given that the idle appeared to become more erratic after swapping the jet.
If this doesn't seem to be a carb issue, i'm going to remove the whole intake (again) and check for possible vacuum leaks. The valve adjustment is also on that list.
EDIT: I just tried swapping to the next richer jet (17.5), and I think it has improved slightly. It's not as bad as it was in the video with the 12.5, but hard to say it's much better than the 15 was. There is still some up and down with the idle, but to a lesser degree. For the sake of science, I want to try going one larger again and see what happens... just concerned by the lack of impact that the mixture screw. Worried it might be pointing to a different issue.
When driving, all is fine until I stop, then the revvs stay at a high idle for a few moments, and then start to drop.
I tried my best to capture a video of it once it was warmed up. This is with the mixture set to 1.5 turns open, and I only touched the throttle to keep it from stalling a couple of times. You can clearly see the idle won't settle down. It's got me wondering it it's a lean condition now, given that the idle appeared to become more erratic after swapping the jet.
If this doesn't seem to be a carb issue, i'm going to remove the whole intake (again) and check for possible vacuum leaks. The valve adjustment is also on that list.
EDIT: I just tried swapping to the next richer jet (17.5), and I think it has improved slightly. It's not as bad as it was in the video with the 12.5, but hard to say it's much better than the 15 was. There is still some up and down with the idle, but to a lesser degree. For the sake of science, I want to try going one larger again and see what happens... just concerned by the lack of impact that the mixture screw. Worried it might be pointing to a different issue.
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